Users of wireless systems for hearing assistance and conference recording presently must choose between radio frequency (RF) systems that emit transmissions that can be monitored by unauthorized listeners, hard-wired large area infrared systems which are not easily transported, or portable infrared systems that do not provide adequate transmission coverage for larger rooms.
A variety of prior systems were developed in the wireless microphone field. For example, U.S. Pat. No. 5,359,448 to Laszlo and Geyer shows a battery powered infrared transmitting device with removable and multiple LED configurations. Other prior systems are shown in U.S. Pat. Nos. 4,229,829 to Grunwald, 5,118,309 to Ford, 5,197,098 to Drapeau, 5,164,984 to Suhami et al., 5,319,805 to Holcomb et al., 4,633,498 to Warnke, and German publication DE 28 20 096 of Weidmann.
RF systems, while readily portable, emit radio waves which can easily be received by unauthorized listeners outside the room in which the transmitter is located. This poses a potentially serious security problem in situations such as legal proceedings and high-level business negotiations.
U.S. Pat. No. 4,831,656 to Southern and Treni, and assigned to the assignee of the present application, describes a wireless conference microphone for use with amplification systems for the hearing impaired which is a predecessor of the present invention. An infrared version of this product has been manufactured by Suffridge & Treni under the trademark Conference-Mate.RTM., most recently as model CM3-95/250. An early version of this model was introduced in April, 1994. It included a jack for attaching an external device with a mute switch and an external output for a modulated audio signal. In November, 1994 an enhanced version of the CM3-95/220 was introduced, which incorporates features of the present invention, providing audio in and audio out lines as well as the mute and external output functions in a multi-functional connector, as will be described in more detail below.
While the April 1994 release of the Conference-Mate.RTM. CM3-95/250 provided improved performance and includes many desirable features, it was constructed using a wooden enclosure and a large number of separate electronic components. The battery was located at the bottom of the unit, so that removal of the circuits was required to change the battery. Further, infrared versions of this device proved difficult to assemble due to the manner in which the LEDs, required for IR transmission, had to be mounted on the sides of the octagonal housing. The LEDs were mounted separately, wired in series, with the string connected by wire to the transmitter printed circuit board mounted on the top surface of the cone. Thus, the bottom portion and the top portion of the enclosure each carried circuit components and were electrically interconnected, making the device difficult to assemble, disassemble, and repair.
Infrared systems offer a secure transmission medium because the lightwave carrier does not pass through opaque surfaces such as walls. However, a problem with currently available infrared systems is that they are either AC-powered hard-wired types which must be permanently installed in a room, or small battery powered units that do not provide adequate coverage for rooms larger than a typical conference room of approximately 1,000 square feet.
Another problem with currently available infrared systems is that many such systems transmit on fixed modulating frequencies or are not readily changeable between frequencies. While 95 kHz has for years been regarded as a world-wide industry standard for infrared hearing assistance systems, the recent introduction of high efficiency fluorescent lighting which causes interference at 95 kHz has prompted some manufacturers to begin offering 250 kHz systems which are not affected by the new lighting systems. This creates a compatibility problem between older 95 kHz equipment and newer 250 kHz systems.
A more ideal wireless system would be configured for easy assembly and service, emit a secure infrared signal, be battery powered for portable operation, be capable of transmitting in a selectable range of frequencies, and be capable of driving secondary infrared emitters so as to provide additional infrared coverage for larger rooms.